In a number of applications, it is necessary to monitor liquids containing significant quantities of suspended materials, which materials may tend to separate or accumulate as a separate phase beneath a body of clearer liquid. Suspended material in liquids tends to absorb, reflect and disperse acoustic energy, although the extent of this effect is frequency dependent. For example, in potable water clarifiers a high frequency is required to detect very small particles in the clear phase. Such high frequency, however, does not work effectively to detect e.g. the density and level of sludge in sewerage treatment plants which, in addition, have several operating states (normal and one or more disturbed), each of them requiring different frequencies. Thus, the effectiveness of a pulse echo ranging system in monitoring such liquids will depend upon the presence of suspended material, including gas bubbles, in the supernatant layer, and the concentration and nature of particles in the layer of precipitated material.
From U.S. Pat. No. 6,898,151 a method of operating a pulse echo ranging system is known, wherein a transducer assembly provides transmission and reception of pulses of high frequency acoustic energy at plural substantially different frequencies. An echo profile is generated for signals received at at least a first of the frequencies, and the signal at another of the frequencies is utilized to enhance the recovery of data beyond that obtained from the first signal alone. If echo profiles are recovered for two or more frequencies, the additional profiles may be utilized to enhance the reliability of recovery wanted data. For example, if supernatant liquid contains large quantities of suspended material, acoustic energy at a frequency best suited to detecting a sludge interface may be so attenuated before reaching the interface that detection is not practicable. In this case, the lower frequency signal may provide additional penetration in order to detect the sludge interface and thus improve reliability. Furthermore, if two or more echo profiles are recovered, the profile recovered from a signal which is not heavily reflected by a sludge interface or other interface which it is desired to detect may be utilized as a reference signal against which echoes occurring in the echo profile from a signal at a different frequency may be assessed. Alternatively, the profiles from different frequencies may be summed, or differenced on the basis that the response from an interface to be detected to signals at substantially different frequencies will be substantially different. If the echo responses at different frequencies are available, it may be possible to deduce characteristics such as quantities of suspended solids, layer density, presence of bubbles, and so on from the different responses to the different frequencies.
In the known method, the received signals are combined to provide enhanced information. The kind of combination depends on the measurement environment and conditions which, however, are often not known.